PDF-XChange is the best compatible pdf reader for Docear. In Linux, I couldn’t manage to open pdf PDF-XChange even I changed it to default pdf reader. But this thread solves this. Basically
“…If you want you can test it for yourself. In Docear, please use “Tools and settings > Miscellaneous > Preferences” to open Docear’s preferences panel. On the “PDF management” tab, under “Open annotations”, select “use compatible PDF viewer”. Then, “Manually define […]” button will be enabled, click it and change the path to the PDF X-Change viewer in the long command to match the path to the PDF X-Change editor’s EXE-file.”